Since an aminoplast obtained by the reaction of a compound bearing an amino group,an acid amide bonding or the like, with formaldehyde, as, for example, melamine resin, urea resin, aniline-formaldehyde resin and the like, has such an active group as active hydrogen, active methylol and active alkoxymethylene, it is generally combined with various resins for coating use having a functional group which is reactive with the abovesaid active group, as hydroxyl group, isocyanate group or the like, including acrylic resin, alkyd resin, polyester resin, epoxy resin, polyurethane resin, polyamide resin, polycarbonate resin and the like, and used as heat curing type coating composition.
Various isocyanate compounds are also used together with such base resins in heat curing type coating compositions. However, such coating compositions generally require high temperature baking, and speaking of the combination of said resins and aminoplasts, are practiced the baking at about 140.degree. C. in the combination of alkyd resin and melamine resin for automobile use and the baking at about 170.degree. C. and more in the combination of epoxy resin and amino resin for coil coating use.
From the standpoint of energy saving, it is, of course, desired to have a low baking temperature and for this end various proposals have been made. One of the proposals heretofore made is to increase an acid value and a hydroxyl number of such resin as alkyd resin, thereby improving the reactivity with aminoplast, and to increase the molecular weight of the resin, thereby improving the curing property thereof. However, this inevitably accompanys with such problems as undesired increase in paint viscosity, lowering in storage stability and decrease in water resistance of the formed film. Attempts have also been made to increase the molecular weight of aminoplast and improve the curing property of the coating composition but the results have been proved to be almost null because of causing additional problem of poor compatibility with alkyd and other resins. There is another proposal of adding an external catalyst as p-toluene sulfonic acid, phosphoric acid and the like to the combined mixture of coating use resin and crosslinking agent. However, no satisfactory results are obtained,giving only poor pigment dispersion stability, causing segregation, due to the presence of external acid catalyst, poor storage stability of the coating composition due to the gradual curing at room temperature and lowering of water resistance of the film. In case that the isocyanate compounds are used as crosslinking agents, employment of external catalyst as tin compound, amino compound and the like, will cause decrease in pot-life of the coating composition, resulting inferior working properties thereof.
From the foregoing, the inventors, having made indeavors for obtaining a coating composition which comprises a coating use resin and a crosslinking agent, has an improved curing property without the necessity of using an external catalyst, is curable at a low temperature or in a shorter period of time, and is excellent in storage stability, capable of resulting excellent film performance, have completed the invention of Japanese Patent Application No. 232900/82 (now publicly opened as Japanese Patent Application Kokai No.124960/84).
In that invention, was provided a coating composition with improved curing property which comprises at least one resin whose acid value based on carboxyl groups of polycarboxylic acid showing a titration midpoint potential of more than -300 mV in a non-aqueous potentiometric titration under the state capable of developing a resinous acid value, is 2 to 50 and which bears functional group being reactive with a crosslinking agent, and at least one crosslinking agent, the weight ratio of said resin to said crosslinking agent being, on solid basis, 95/5 to 45/55.
However, in the latest automobile and other industries, the so-called metallic coating composition containing aluminium powder or the like is often applied on an intermediate or top coat layer. In such a metallic coating, it is very common to adopt the so-called two-coat-one-bake coating system wherein a metallic coating composition containing metallic powder is first applied, followed by a clear coating, and thus obtained coatings are then baked in one step.
However, there are many cases where white blooming and blisters are appeared on the coating exposed to lights and especially ultra violet rays or rain falls. This is believed to be due to the fact that said ultra violet rays or rain water will penetrate through the metallic coating and acts directly upon the underlaying intermediate or top coat, thereby causing interface adhesive failure and then white blooming or blisters of the metallic coating.
Therefore,in the related technical fields, public attentions are directed to the importance of weather resistance against transmitted light of such intermediate or top coat layer. In the aforesaid Japanese Patent Application No. 232900/82, such aromatic acids as phthalic anhydride, pyromellitic acid, pyromellitic anhydride, trimellitic acid, trimellitic anhydride, tetrachlorophthalic anhydride, tetrachlorophthalic acid, tetrabromophthalic anhydride, tetrabromophthalic anhydride, HET acid, HET anhydride and the like are intended as the polycarboxylic acid showing the required titration midpoint potential, in a non-aqueous potentiometric titration under the state capable of developing the resinous acid value, of more than -300 mV and a resin having an acid value based on carboxyl groups of such aromatic acid of 2 to 50 is combined with an aminoplast resin or the like to obtain a resinous composition which is curable at a low temperature without the necessity of using an external catalyst.
However, subsequent studies have revealed the fact that if a resinous acid value based on such acid is too excessively high, the weather resistance against transmitted light of the resulted resinous composition will get worse on the contraty. For example, in the case of a resin containing as a part of acid components, tetrachlorophthalic anhydride, if a resinous acid value based on said particular acid exceeds over 8, there is a tendency that weather resistance against transmitted light of the resulted resin will be extremely lowered.
For the purpose of obtaining improved curing properties at a low temperature, the resinous acid value based on the aforesaid polycarboxylic acid should be as high as possible, which , however, is not desired from the standview of obtaining an improved weather resistance against transmitted light of the resin. Thus, the both requirements are in conflict with each other.
In the case where a sulfonic acid is added to a resinous composition for coating use as an external catalyst, there are no particular problems in both respects of low temperature curing porperties and weather resistance against transmitted light , and therefore, it is quite natural that one may try to use this type of external catalyst in a resinous composition of Japanese Patent Application No. 232900/82 to make up a deficient resinous acid value for the improvement in the weather resistance against transmitted light.
However, since a sulfonic acid must, in general, be added in a considerable quantity to a resinous composition for the intended objects, additional problems of lowering in pigment dispersion stability and storage stability are always accompanied therewith.
It is, therefore, an object of the invention to provide a resinous composition for coating use which is useful in an intermediate or top coat paint and is excellent in low temperature curing properties weather resistance against transmitted light, pigment dispersibility and storage stability of the coating composition.